1. Field of the Invention
The present invention relates to a lead frame to be used for fabrication of a plastic-encapsulated semiconductor device, more particularly to a lead frame for a semiconductor device and a photocoupler which are made by transfer molding.
2. Related Art
In general, the fabrication process of a plastic-encapsulated semiconductor device comprises the step of attaching a chip on a lead frame, the step of electrically interconnecting the attached chip to leads by wire bonding, the step of encapsulating the wire-bonded chip in plastic by transfer molding, and the step of cutting tie bars. One of the functions of these tie bars is to prevent resin bleed during the transfer molding step.
In the tie bar cutting step, while the lead frame is pressed and fixed by a tie bar cutting die, the tie bars and thick flashes (resin masses which accumulate in gaps between the plastic package and the tie bars) are punched out by a punch (see for example Japanese Patent Laid-open Publication No. H7-221245)
This tie bar cutting operation needs to allow for the influence of cracking or the like on the molded package (hereinafter called plastic package), wear of the punch, and any other factors. Hence, the tie bars should be spaced from the plastic package by 0.15 mm to 0.2 mm or greater, in order to make up for misalignment and margins of the molding die. Such clearances permit the punch to sever the tie bars.
In the transfer molding step, plastic is filled into a molding die which is closed on the lead frame. As known, where the plastic package and the tie bars are spaced by a greater clearance, the clamping pressure at the surface of the molding die gets lower. To prevent resin from bleeding beyond the tie bars, high pressure (generally, as high as several hundred tons) has to be applied to the molding die.
If a photocoupler is made by double transfer molding, two rows of tie bars are necessary, as first tie bars for the primary molding operation and second tie bars for the secondary molding operation. The first and second tie bars are cut after the respective molding operations, which naturally necessitate equipment and clearances for the two cutting operations.
FIG. 9 is a plan view which shows the structure of conventional photocouplers, after the molding step in the double transfer molding technique. FIG. 10 is a vertical section view which shows the structure (typical structure) of a finished face-to-face type photocoupler.
The photocoupler has a pair of lead frames (an emitter-side lead frame 100a and a detector-side lead frame 100b) which comprise frame headers 11a, 11b which respectively mounts a light-emitting element 17 and a light-detecting element 18 (not shown in FIG. 9) a plurality of inner leads 12a, 12b which are arranged in the periphery of these frame headers at a predetermined interval, a plurality of outer leads 13a, 13b which correspond to the inner leads 12a, 12b, and tie bars 14a, 14b which connect the inner leads 12a, 12b and the outer leads 13a, 13b. The pair of lead frames 100a, 100b are matched such that the light-emitting element 17 is vertically face to face with the light-detecting element 18. As shown in FIG. 9, a plurality of such pairs are connected together via a frame rail unit composed of emitter-side frame rails 101a and detector-side frame rails 101b. The frame headers 11a, 11b and the inner leads 12a, 12b which are arranged in their periphery are encapsulated in plastic to form a plastic package 15.
Additionally, numerals 31 in FIG. 9 indicate reference holes to be relied on when the frame rails 101a, 101b are matched or placed into a mold.
As a traditional requirement, sufficient clearances P11 (0.15 mm to 0.2 mm or greater) are secured between the plastic packages 15 and the tie bars 14a, 14b, allowing for the influence of cracking or the like, wear of the punch, and any other factors which may be anticipated when the tie bars 14a, 14b are cut off by a punch.
According to this structure, when the emitter-side lead frames 100a and the detector-side lead frames 100b are vertically matched on one another, the light-emitting elements 17 are opposed to the light-detecting elements 18. In the structure illustrated in FIG. 9, the outer leads 13a of the emitter-side lead frames 100a and the outer leads 13b of the detector-side lead frames 100b are arranged to alternate in the horizontal direction without overlapping each other. Moreover, to improve the mounting density, the outer leads 13a, 13b are staggered.
As another traditional requirement, anti-interference clearances P12 are secured between the tips of the outer leads 13a, 13b and the tie bars 14b, 14a which face the outer leads 13a, 13b, respectively.
Thus, when the tie bars and thick flashes are punched out by a punch after the molding step according to the conventional transfer molding technique, sufficient clearances P11 are indispensable between the plastic packages 15 and the tie bars 14a, 14b for the purpose of eliminating the influence of cracking or the like on the plastic package. It is also essential to leave anti-interference clearances P12 between the tips of the outer leads 13a, 13b and the opposed tie bars 14b, 14a. Such clearances hamper improvement of the mounting density of lead frames and thus limit the number of photocouplers per frame rail unit. However, a simple attempt to locate the tie bars closer to the plastic package will create other problems. For example, errors due to misalignment of the molding die or wear of the punch disable the tie bar cutting operation, or leave noticeable burrs on the tie bars.
There is still another problem relating to transfer molding. To avoid resin bleed or the like in the situation where plastic is filled into a molding die which is closed on the lead frame, high pressure is applied to the molding die by means of a high-pressure press. Nevertheless, the high-pressure press is so expensive as to boost the equipment cost and the production cost.